The capillary water absorption behavior of cement-based materials deviates progressively from the classical unsaturated flow theories due to specific interactions between water and cement hydrates. Although the impact of supplementary cementitious materials on the initial capillary sorptivity has been widely investigated, their influence on anomalous capillary absorption behavior need further exploration. By taking the technical advantage of low-field nuclear magnetic resonance (LF-NMR), this study investigates the long-term capillary absorption of water and isopropanol (IPA) into white cement mortars with high content of fly ash, slag, and silica fume.IntroductionThe durability of cement-based materials is significantly influenced by the transport processes of water, gases such as CO₂, and ions like Cl^-. Water transport, particularly capillary water absorption, is crucial for durability performance. In most engineering practices, cement-based materials are non-saturated, making capillary water absorption the primary mode of water transport. This process is driven by capillary pressure arising from the meniscus within pores and is governed by the extended Darcy's law or the Richard's equation. In one-dimensional capillary absorption scenarios, theoretically there exists a linear relationship between the cumulative absorbed volume per unit area and the square root of absorption time, which is known as the square root of time linear law. The slope obtained from linear fitting is defined as the capillary sorptivity, which serves as a quantitative indicator of durability. However, long-term capillary water absorption often deviates from this linear law due to specific physicochemical interactions between water and cement hydrates, especially calcium silicate hydrate (C-S-H) gel. The widespread use of supplementary cementitious materials such as fly ash, slag, and silica fume alters the composition and micro-structure of C-S-H gel through secondary hydration reactions, significantly impacting the properties of cement-based materials. Although the capillary sorptivity has been well recognized as a durability indicator, the anomalous phenomena observed during long-term capillary water absorption are frequently overlooked, leading to incomplete research and inconclusive findings. This study focuses on the long-term capillary absorption processes of water and IPA into cement mortars with representative supplementary cementitious materials, including fly ash, slag, and silica fume. The aim is to elucidate the influence mechanisms of supplementary cementitious materials on the capillary absorption properties of cement mortars.MethodsThe experimental section focused on investigating the impact of supplementary cementitious materials on the capillary absorption behavior of cement mortars. Samples were prepared with sands, white cement and high contents of fly ash, slag, and silica fume. These specimens were subjected to both standard curing at 20 ℃ and accelerated curing at 60 ℃ to assess the influence of curing temperature. Testing methods encompassed capillary absorption tests using IPA and water, along with low-field magnetic resonance relaxation technique to analyze pore structure evolution. The capillary sorptivity were determined by fitting the experimental data to a square root of time linear law, and deviations from this law were analyzed to characterize water sensitivity. Data analysis involved examining changes in capillary sorptivity, deviation times, and pore structure characteristics to elucidate the mechanisms by which supplementary cementitious materials affect the capillary absorption properties of cement mortar.Results and discussionThe incorporation of fly ash, slag, and silica fume significantly alters the capillary sorptivity of both IPA and water into cement mortar. Specifically, all three supplementary cementitious materials reduce the capillary sorptivity of IPA, with fly ash exhibiting the most pronounced effect. During the initial stage of capillary absorption of water, slag increases the sorptivity, while silica fume and fly ash decrease it. In the later stage, high silica fume content notably enhances the sorptivity, whereas high slag and fly ash contents decrease it. The water sensitivity of cement-based materials is also affected. High slag and fly ash increases the water sensitivity of cement mortars, leading to earlier deviation from the initial linear law and higher degrees of deviation in later stages. These findings indicate that supplementary cementitious materials modify the composition and microstructure of C-S-H gel, thereby affecting the water sensitivity and durability of cement-based materials.ConclusionsThe addition of three supplementary cementitious materials will reduce the capillary sorptivity of isopropanol, and the reduction effect of fly ash is most significant. In the initial stage of capillary absorption of water, slag significantly increases the initial sorptivity of mortar, while silica fume and fly ash show inhibiting effects. In the later stage, silica fume significantly increases the secondary sorpvitity of water into mortar, while slag and fly ash both decrease their secondary sorptivity. The addition of slag and fly ash will significantly increase the difference between the initial and secondary capillary sorptivity. High content of silica fume can reduce the water sensitivity of cement-based materials, while high content of slag and fly ash can improve the water sensitivity, so as to advance the deviation of capillary absorption of water from the square root of time linear law and to increase the degree of deviation. The improvement of water sensitivity is the most significant for the mortar with high slag substitution. Curing at high temperature slightly reduces the water sensitivity of mortar with high slag content, but almost has no effect on the water sensitivity of mortars with high silica fume and fly ash contents.